This invention pertains generally to systems and methods for transmitting signals such as human speech and more particularly to a system and method for transmitting such signals in a bandwidth less than the frequency range found in the signals.
Cummunication systems such as telephone systems, radio systems and other systems for conveying intelligence commonly have only a limited bandwidth or frequency range available in which the signals can be transmitted. By reducing the bandwidth required to transmit individual signals, the number of signals which can be transmitted in a given bandwidth can be increased and interference between adjacent channels can be avoided.
In the past, it has been recognized that with signals having components occupying a range of frequencies, the different frequency components generally are not present simultaneously, but are separated temporally. The most significant portion of human speech, for example, generally falls within a range of about 200-3200 Hz, with the lower portion of this range (below 1600 Hz) being predominantly occupied by vowel sounds and the upper portion (above 1600 Hz) being occupied mainly by consonant sounds. These two classes of sounds generally do not occur simultaneously, and when a large amount of energy is present in one class, there will be very little energy in the other. Vowels sounds generally occur at a level on the order of 10-15 db higher than consonant sounds, although consonant sounds carry about 80% of the speech information content.
Heretofore, there have been attempts to utilize the temporal separation and amplitude differences of vowel and consonant sounds to permit time sharing transmission of the two classes of sound in the bandwidth normally occupied by one. Examples of such systems are found in U.S. Pat. Nos. 1,836,824 and 2,726,283. These systems have certain limitations and disadvantages such as high switching noise and synchronization problems which have made them unsuitable for commercial use.